This invention relates generally to tightening and tightening control systems, and more particularly to systems for tightening fasteners which exhibit more than one installation region during a complete tightening cycle, such as, for example, blind fasteners.
In order to properly install a blind fastener into a workpiece hole, a first force or torque value must be reached in order to form the bulb, and a final tightening force or torque must be applied in order to tighten the fastener to the desired clamp load. These force or torque values may be referred to, respectively, as the bulb forming value and the final tightened value. The blind fasteners generally contemplated herein are of two types. The first type is a conventional two piece blind fastener including a pulling pin having an enlarged head at the blind (or inaccessible) end, and a tubular sleeve which is carried on the pin and is upset at the blindside workpiece surface when the pin is pulled in an axial direction, which causes the head to exert an axial force on the end of the sleeve. Examples of two-piece blind fasteners can be found in U.S. Pat. Nos. 3,253,495; 2,030,167; and 2,030,169. A second conventional type consists of three pieces including a pulling pin carrying a tubular sleeve, and a tubular expander having a tapered nose portion at one end. The expander is also carried on the pin with the tapered nose portion adjacent the sleeve. When an axial force is exerted on the pin by pulling or torquing, the sleeve advances over the tapered end of the expander and is radially outwardly expanded until the sleeve contacts the blindside workpiece surface and then forms an outwardly flared bulb. Examples of three-piece blind fasteners can be found in U.S. Pat. Nos. 3,643,544; 3,369,442; 3,063,329; 3,277,771; and 3,107,572. It should be noted that blind fasteners may generally be installed by pulling in an axial direction as shown in U.S. Pat. Nos. 3,253,495; 2,030,167; 2,030,169; 3,277,771; and 3,107,572, for example, or by exerting torque to move the pin in an axial direction as shown in U.S. Pat. Nos. 3,443,474; and 3,657,956.
Presently, there are no known adaptive automatic assembly tools for installing blind fasteners to a desired tightened condition responsive to the particular fastener assembly and joint being encountered. Installation tools such as shown in U.S. Pat. Nos. 3,643,544; 3,369,442; and 3,063,329 can install blind fasteners to a preload generally controlled by the breaking strength of a breakneck groove in the pulling pin. The breaking strength of this groove is dictated by the anticipated force necessary to form the bulb, and consequently this breaking strength must be greater than the bulb forming force. It should further be noted that the bulb forming force can vary with the grip length of the joint, friction in the joint/sleeve configuration and material, manufacturing tolerances, and squareness of the blindside surface with the axis of the fastener. Therefore, the breakneck strength must be established sufficiently high to accommodate these variables interacting to generate the highest bulb forming force. As a result, this force can have deleterious effects when the same variables interact to generate low bulb forming forces. Consequently, performance compromises of the fastener usually result. Additionally, there is also a problem of relaxation or recovery of clamping load in the joint after the pin breaks, causing inconsistencies in preload values from joint to joint in a structure. These and other problems are overcome by the present invention.
Referring to FIG. 1, the two force or torque vs. stroke or rotation curves shown for a typical blind fastener represent extremes of physical conditions which could be encountered in two separate joints in the same or different workpieces. No single force or torque value satisfies both conditions. For example, if the breaking strength of the pulling pin is set at a value corresponding to [(F.sub.S)B], fastener B may be tightened to the correct clamp load but this value will not be sufficient to form the bulb in Fastener A. Conversely, if the breaking strength of the pin is equal to a value [(F.sub.S).sub.A ], undesirable formation or failure of the bulb forming member could occur in fastener B. It is this type of problem which has placed limitations on allowable joint and/or fastener manufacturing tolerances, resulting in increased costs to both the users and manufacturers of high performance blind fasteners. No automatic installation or tightening tools are currently known which can overcome these problems. These and other problems are, however, overcome by the present invention.